The present invention relates to a method of marking polymer-based laminates used during the production of documents such as for example identification cards, credit cards and banknotes.
Identification cards, credit cards etc. as well as some banknotes are often formed of polymer-based laminates. Marking these cards and banknotes is common practice to deter forgery and allow counterfeit cards and banknotes to be readily detected. Ideally, these markings are placed within the laminates without damaging their skins so that security features do not have to be incorporated into the manufacturing process of the polymer. Many techniques have been devised to mark cards using laser radiation to satisfy the above.
For example, U.S. Pat. No. 4,507,346 to Maurer et al discloses a multi-layer identification card and method of making the same. The identification card includes an inlay on which a thermosensitive coating is applied. A synthetic layer is disposed on the inlay and on the thermosensitive coating and includes a blowing agent. A laser beam recorder is used to activate the blowing agent and the thermosensitive coating to personalize the identification card.
U.S. Pat. No. 4,597,592 to Maurer et al discloses a multi-layer identification card with duplicate data. The identification card includes a backing, an opaque middle layer and a cover film. Characters are burned primarily in the opaque middle layer using a laser.
U.S. Pat. Nos. 4,544,181 and 4.579,754 to Maurer et al disclose a multi-layer identification card including an inner opaque layer surrounded by upper and lower cover sheets. A photograph and two data areas are provided on the inner opaque layer. A magnetic strip is laminated on the under surface of the lower cover sheet. The inner opaque layer may be coated with a thermosensitive dye. Alphanumeric indicia is printed on the inner opaque layer by burning or blackening the inner opaque layer using a laser beam.
As will be appreciated, since the above described techniques use lasers to mark cards, the marking techniques are non-contact, fast and environmentally safe due to the fact that no consumables are involved in the marking process. However, improvements to marking techniques are continually being sought.
It is therefore an object of the present invention to provide a novel apparatus and method of marking a polymer-based laminate.
According to one aspect of the present invention there is provided a method of marking a polymer-based laminate including a core layer, a cover layer on at least one side of said core layer and an adhesive layer acting between said core layer and said cover layer, said core, adhesive and cover layers having different radiation transmission coefficients, said method comprising the step of:
selectively irradiating said laminate with laser radiation having a fluence sufficient to mark said adhesive layer at selected locations while maintaining said cover layer intact.
Preferably, the fluence is in the range of from about 0.06 mJ/cm2 to 0.1 2 mJ/cm2. During the step of irradiating, it is preferred that the selected areas of the cover layer are irradiated with at least one pulse of laser radiation and that the laser radiation is patterned prior to irradiating the cover layer. It is also preferred that the spatial distribution of the laser radiation is adjusted prior to irradiating the cover layer.
According to another aspect of the present invention there is provided an apparatus for marking a polymer-based laminate comprising:
a source of laser radiation to generate a beam of laser radiation having a fluence in the range of from about 0.06 mJ/cm2 to 0.12 mJ/cm2;
a beam adjuster to adjust the spatial distribution of said beam; and
a focusing lens to focus said adjusted beam onto a polymer-based laminate to be marked.
Preferably, the beam adjuster includes a beam homogenizer and a field lens. It is also preferred that the source is an Excimer laser and wherein the beam of laser radiation has a wavelength equal to about 248 nm.
According to yet another aspect of the present invention there is provided a polymer-based laminate comprising:
a core;
a cover surrounding said core; and
adhesive acting between said core and said cover, wherein said core, adhesive and cover have different radiation transmission coefficients and wherein said adhesive includes at least one marking thereon resulting from exposure of said laminate to laser radiation having a fluence sufficient to mark said adhesive while maintaining said cover intact.
The present invention allows polymer-based laminates used to form valuable documents, to be marked in a secure, non-contact manner after the polymer manufacturing process. This is achieved by marking the adhesive acting between the core and the skin of the laminate using laser radiation.
An embodiment of the present invention will now be described more fully with reference to the accompanying drawings in which:
FIG. 1 is a cross-sectional view of a polymer-based laminate to be marked using laser radiation;
FIG. 2 is a schematic diagram of an apparatus for marking polymer-based laminates in accordance with the present invention;
FIG. 3 is a graph showing the spectra of various polymer-based laminates exposed to Excimer laser radiation;
FIG. 4 is a table showing threshold and damage fluences for some of the polymer-based laminates illustrated in FIG. 3;
FIG. 5 illustrates a polymer-based laminate marked with bar codes using the apparatus of FIG. 2; and
FIG. 6 illustrates a polymer-based laminate marked with lettering using the apparatus of FIG. 2.